An artifact-robust framework for measuring tCS effects during stimulation

Objective: Transcranial current stimulation (tCS) is a promising technique to non-invasively modulate human brain activity. However, stimulation artifacts in EEG and MEG recordings severely hinder the study of its online effects. Approach: Here, we introduce a new approach to account for these artifacts. The approach rests on two key ideas. First, we focus on interactions of tCS with intrinsic brain activity, which are absent for tCS artifacts. Second, rather than removing artifacts, we compare composite signals that share similar artifacts but potentially differ in the interaction of tCS with intrinsic brain activity. We follow a simple logic: if tCS does not interact with intrinsic brain activity, then neural activity during simultaneous sensory stimulation and tCS should equal the linear superposition of the neural effects of each applied alone. Any deviation from this prediction provides evidence for a neural interaction. Main results: We tested this approach in a proof-of-principle MEG study, applying 10 Hz transcranial alternating current stimulation (tACS) during rest and during a 10 Hz visual flicker. We compared neural activity during simultaneous stimulation with that predicted by the linear superposition of flicker and tACS alone and found a phase-dependent interaction between tACS and flicker-evoked brain activity. Significance: Our work establishes a novel approach to investigate online effects of tCS and suggests a state-dependent interaction of tACS with human brain activity.